Purification of wood pulp



Sept. 1945- w. R. COLLINGS ETAL 2,385,259

PURIFICATION OF WOOD PULP Filed May 10, 1943 Flow Sheet Na 0// (Solid or Cone.)

A B C Sleeping lazf was/7 2 12d. was/1 u\l liquor .sforage liquor sforage liquor .slorage Chi ea u/ Final was/1 wafer Sleeping Ta n/c M far 'r In? eff/aen/ E 0 ia l yzer Alkali soluble impur/f/es l'o wast:

2nd. effluen/ 5rd. eff/uenf Fina/ efl'luem f0 was/e Purified gale l Marlin J. Raberfis ATTORNEYS Patented Sept. 18, 1945 PURIFICATION OF WOOD PULP William R. Collings, Richard D. Freeman, and Martin J. Roberts, Midland, Mich, and Willis 0. Hisey, Syracuse, N. Y., assignors to The Dow Chemical Company, Midland, Mich, a corporation of Michigan Application May 10, 1943, Serial No. 486,346

2 Claims. (Cl. 92-13) This invention relates to a process for the puriilcation or wood pulp to increase its content of alpha-cellulose.

The necessity of using alpha-cellulose of high purity in the preparation of cellulose derivatives, such as esters and ethers, is well understood in the art. Usually a product containing at least 96 to 98 per cent of alpha-cellulose is required to form a satisfactory derivative and the preparation of such high purity alpha-cellulose from wood pulp is of great importance commercially.

Two general methods of treating or purifying wood pulp to increase its alpha-cellulose content have been described. In one of these processes, usually referred to as the hot process, the pulp is digested at from 50 to 150 C. with from 6 to 15 per cent of its, weight of sodium hydroxide in the form of a 1 to 2 per cent aqueous solution and the alkali subsequently washed from the pulp with water. The hot process is, however, not applicable to the purification of sulfate or soda pulp. Furthermore, in the case of sulfite pulp, the yield of alpha-cellulose is low when attempts are made to prepare a product of high purity. The loss of cellulose is particularly great and the process must be carried out at the higher temperatures under pressure when alpha-cellulose of purity greater than about 94 per cent is desired. Also the recovery of the alkali used may not be efiected economically due to its high dilution. Consequently, the hot process is not well adapted to the economical production from wood pulp of the high purity alpha-cellulose required for the preparation of cellulose esters and ethers.

In the other general process, usually referred to as the cold process, the pulp is steeped at a temperature below 50 C. with from 40 to 225 per cent of its weight of sodium hydroxide in the form of a 5 to 20 per cent aqueous solution. Although the cold process may be adapted to the preparation of a high purity alpha-cellulose from either sulfate, soda, or sulfite pulp, it has heretofore been possible to operate the process economically only where the relatively large quantity of highly impure sodium hydroxide solution obtained as efliuent liquor from the process can be used directly in some other process without purification or recovery, e. g. by adding sodium sulfide to the efliuent liquor and using the resulting solution in cooking wood to make sulfate pulp. The necessity of profitably utilizing the impure caustic soda from the cold process has thus limited greatly the practical application of the process.

It is, therefore, an object of the present invenof high purity may be prepared conveniently and economically.

An additional object is to provide a cold process for the preparation of high purity alpha-cellulose from wood pulp wherein a major proportion of the eflluent alkaline liquor may be recycled.

An additional object is to provide a process whereby alpha-cellulose of at least 97.0 per cent purity may be prepared economically and in high yield from wood pulp.

These and related objects may be accomplished readily by soaking or steeping chipped wood pulp in from 8 to 15 per cent cold aqueous sodium hydroxide to swell the fibers and to dissolve the coldalkali-soluble constituents of the pulp and subsequenitly washing the steeped pulp with succeeding portions of aqueous sodium hydroxide of decreasing sodium hydroxide content to wash out the dissolved impurities while the fibers are in a swollen condition, and finally with water to wash but substantially all of the dilute sodium hydroxide wash liquor. The eflluent liquors from the steeping and washing steps are collected in several portions of decreasing sodium hydroxide content and certain of these portions are recycled to the process in a certain order, the volumes of the steeping liquor and of the several portions of wash liquor and eflluent liquor recycled being maintained within certain ranges of proportions as will be hereinafter described.

The chipped pulp referred to is obtained by chopping sheeted pulp into small pieces rather than by shredding it into a flufiy mass. Machinery for chipping the pulp in this manner is available in the industry. The use of chipped pulp rather than of a flufiy, shredded pulp permits 0! a more even steeping and washing of the pulp because there is little tendency for the liquor to run in channels through the bed of chipped pulp. When a fluflfy, shredded pulp is used considerable channeling occurs and only very uneven and incomplete washing of the impurities from the pulp is obtained. During the last of the washing steps considerable shrinkage of the pulp occurs due to the reduced alkali content of the wash liquor and this leads to further channeling and even to withdrawal of the pulp mass for a considerable distance away from the walls of the steeping tank when a flufiy pulp is used. However, when chips are used such shrinkage does not alter materially the evenness of flow of the washing liquor through the pulp bed since the mass merely settles somewhat with little or no tendency to channel or to shrink away from the walls of the steeping tank.

tion to provide a process whereby alpha-cellulose It also appears that the penetration of the original steeping liquor through the chipped pulp is somewhat more even and uniform than it is through a fluffy pulp'mass. Sheeted pulp chipped into pieces approximately 0.5 by 0.25 inch has been used to advantage but it is apparent that larger or smaller chips may be used, if desired. It should be noted thatthe term consistency is used herein to denote the proportion of chipped pulp to total charge in the steeping tank even though the pulp is in the form of chips and is not dispersed to form a uniform slurry, such as is the case in many instances where the term consistency is used.

The accompanying drawing is a flow sheet of the process. The steeping tank D may be of any convenient design and may be fitted with a false bottom or other conventional means whereby the draining of liquid away from the pulp at the bottom of the tank is facilitated. As indicated in the fiow sheet chipped wood pulp is loaded into the steeping tank followed by steeping liquor from a steeping liquor storage A. The steeping liquor and the chipped pulp are used in such proportions as to give a consistency of from 7 to per cent, preferably from 10 to 12 per cent, in the steeping tank.

After steeping has progressed to the desired degree, the steeping liquor containing cold-alkali-soluble impurities dissolved from the pulp is drained from the tank and at the same time a first wash liquor, which contains a lower concentration of sodium hydroxide than the steeping liquor, is run into the tank onto the pulp bed from a first wash liquor storage B so as to maintain the liquid level in the steeping tank substantially unchanged. The first wash liquor is followed by a second wash liquor, of lower sodium hydroxide content than the first wash liquor, added in the same manner from a second wash liquor storage C, and the pulp is finally washed with sumcient fresh water to .wash out substantially all of the sodium hydroxide.

The effluent from the steeped pulp is, as previously mentioned, collected in several separate portions. The first portion, which consists largely of the used steeping liquor contaminated with cold-alkali-soluble substances dissolved from the pulp, is highly impure and is passed through a dialyzer E to effect a separation of the sodium hydroxide from the dissolved impurities. The methods and apparatus for dialyzing impure sodium hydroxide solutions to obtain relatively pure sodium hydroxide are well known in the art and need not be discussed here. Such dialysis is practiced commercially in the viscose industry. The purified sodium hydroxide solution obtained from the dialyzer is stored in the first wash liquor storage B and is subsequently used as the first wash liquor for a. new batch of pulp. The separated impurities are discarded.

The second portion of the effluent collected from the steeped pulp, which consists largely of the last runnings of the used steeping liquor mixed with a considerable proportion of the first wash liquor from the batch and which has a lower sodium hydroxide content than the first portion collected, is enriched with solid or concentrated sodium hydroxide and stored in the steeping liquor storage A. It is subsequently used as steeping liquor for a new batch of pulp. The third portion of eflluent collected from the steeped pulp consists of a still more dilute sodium hydroxide solution than the second portion and contains a relatively small amount of impurities dissolved from the pulp. The third portion of the efiiuent is stored in the second wash liquor storage C and subsequently used as the second wash liquor for a new batch of pulp. The

remaining effluent from the washed pulp has too low a sodium hydroxide content to be re-used economically in the process and is usually discarded.

The purified pulp, which has an alpha-cellulose content of at least 97.0 per cent on the dry basis, may be further treated in any desired manner to convert it to a form usable for any desired purpose. When it is desired to repare a cellulose ester or ether, the purified pulp may be sheeted and dried and subsequently treated with esterifying or etherifying agents in conventional manner. The invention is, however, not restricted by thefmanner in which the steeped and washed pulp is subsequently treated or utilized.

The economical operation of the process on a commercial basis depends to a considerable extent upon the maintenance of a balance between the volumes and concentrations of the steeping liquor, the wash liquors and the separately collected portions of effluent liquor. As mentioned previously, the steeping liquor may contain from about 8 to about 15 per cent of sodium hydroxide. Generally speaking, less concentrated solutions may be used at low temperatures than are desirable at ordinary or room temperatures. However, it is usually more convenient and preferable to carry out the steeping operation at a tem perature of from 0 to 30 C. using a steeping liquor containing from 10 to 13 per cent of sodium hydroxide. When using a steeping liquor of such concentration the pulp is rapidly and thoroughly swollen allowing the liquor to permeate it thoroughly and to come in contact with and to dissolve substantially all of the coldalkali-soluble impurities therein.

An additional advantage in using a steeping liquor of the preferred sodium hydroxide content lies in the fact that the steeped pulp may be washed with portions of more dilute sodium hydroxide solution, such as may be recovered and recycled from a previous batch, without substantial shrinkage of the pulp, thus permitting a high removal of the alkali-soluble impurities with a minimum of washing. When a steeping 'liquor having a sodium hydroxide content lower than the preferred range is used considerable shrinkage of the pulp occurs during the washing with the more dilute wash liquors which may be recovered and recycled from a previous batch and a considerable amount of the alkali-soluble impurities may be reprecipitated or entrapped among the pulp fibers and may be removed only with great difficulty if at all. This difliculty, which is particularly serious when the steeping liquor contains less than 8 per cent of sodiumhydroxide, may, in some instances, be overcome by using a first wash liquor having about the same concentration of sodium hydroxide as the steeping liquor but such wash liquor cannot be recovered and recycled from a previous batch and must be obtained either by enriching a recycled portion of efiiuent from a. previous batch or by making up a fresh sodium hydroxide solution for use as wash liquor. Either of these alternatives requires the use of additional sodium hydroxide and thus renders the process less economical to operate.

An additional advantage of using a steeping liquor of the preferred sodium hydroxide content lies in the fact that it furnishes a more concentrated dialyzer feed liquor. It is well known that the economy of operation of a dialyzer decreases rapidly as the concentration of sodium hydroxide in the solution being dialyred is lowered. Emuent obtained from the present process using a steeping liquor of the preferred sodium hydroxide content may be dialyzed more economically than is the case when a more dilute steeping liquor is used. When the steeping liquor contains less than about 8 per cent of sodium hydroxide, dialysis of the eflluent liquor becomes uneconomical and impractical and the portion of the effluent highest in dissolved impurities, and, correspondingly, highest in sodium hydroxide content, cannot be purified and recycled to the process, but must be discarded.

The pulp is caused to remain in contact with the steeping liquor until substantially all of the cold-alkali-soluble impurities, e. g. beta-cellulose, gamma-cellulose, and pentosans, have dissolved. Although the length of the steeping period depends to some extent upon the temperature and the concentration of caustic soda in the steeping liquor, steeping is usually carried on for from 0.5 to 3 hours and the pulp then washed twice. When the volume of the steeping liquor is taken as unity, about one-half volume of the first wash liquor and about one volume of the second wash liquor are used. The sodium hydroxide content of the first wash liquor is from about 4 to about 6 per cent and that of the second wash liquor is from about 1.5 to about 2.5 per cent. The pulp is then finally washed with fresh water until substantially free of sodium hydroxide, from 2.5 to 4 volumes of water being usually required.

Three separate portions of effluent liquor are collected. The first portion consisting of about one-half volume contains from about 7 to about 14.5 per cent of sodium hydroxide, usually from about 9 to about 12.5 per cent when a steeping liquor of the preferred sodium hydroxide content is used, together with impurities dissolved from the pulp. The first portion of efliuent is dialyzed and there is obtained from the dialysis step about one-half volume of purified solution containing from about 4 to about 6 per cent of sodium hydroxide. The purified solution is used for the first wash liquor of a succeeding batch. The second portion of efliuent liquor collected consists of about one volume of solution containing from about 6 to about 8 per cent of sodium hydroxide. Although the second portion of eflluent contains a; considerable amount of the alkalisoluble impurities dissolved from the pulp, it may, after enrichment with solid or concentrated sodium hydroxide, be used as steeping liquor for a succeeding batch. The third portion of the efliuent liquor which is collected consists of about one volume of solution containing from about 1.5 to about 2.5 per cent of sodium hydroxide. The third portion is used without further treatment as the second wash liquor for a succeeding batch of pulp. Further efiiuent from the steeping tank is discarded.

The process has a number ofyv'adyantages over the cold processes previously de ribed. A large proportion of the sodium hydroxide-used may be recycled to the process and it is thusnot necessary to operate the process in conjunction with some other process in which the eiliuent may be used. The .total amount of sodium hydroxide consumed in the process usually amounts to from 25 to 50 per cent of the weight of the pulp purified. The production of alpha-cellulose of high purity is facilitated by washing the steeping liquor from the pulp with portions of more dilute sodium hydroxide and gradually decreasing the sodium hydroxide content of the successive portions. rather than by washing the steeped pulp directly with pure water. The washing of the steeped pulp with a first wash of substantially pure sodium hydroxide solution from the dialyzer leads to a more effective removal of impurities from the still highly swollen pulp than is possible when one of the effluent portions of equivalent sodium hydroxide content but containing a substantial proportion of dissolved impurities is used. More nearly complete removal of the dissolved impurities is thus obtained when the collected portions of efliuent are recycled in the described manner than is possible when they are recycled in "counter-current manner, e. g. than when the dialyzed first efliuent portion is enriched with sodium hydroxide and used as steeping liquor for a new batch of pulp and the second and third efliuent portions then used as the first and second wash liquors, respectively, for the new batch. The process is also superior to the hot processes heretofore described in that it is applicable to bleached, semi-bleached or unbleached sulfate and soda pulp as well as to sulfite pulp and may be operated to produce a purified pulp having an alpha-cellulose content greater than 97.0 per cent.

Although the process has been described as a batch process involving the use of one steeping tank and appropriate storage tanks, it is apparent that a number of steeping tanks may be employed and the various effluent portions recycled to the several steeping tanks in appropriate manner thus rendering the process semi-continuous in operation.

It is also apparent that considerable latitude is permissible in the relative volumes of the washing liquors and of the recycled portions of efliuent liquor without materially reducing the alpha-cellulose content of the purified pulp, although for most economical operation of the process the relative volumes of the washing liquors and recycled eiliuent portions should not vary more than about 15 to 20 per cent from those mentioned in the foregoing description.

Certain advantages of the invention may be seen from the following example which is given by way of illustration only.

Example 687 pounds (oven dry basis) of chipped sulfite pulp containing about 5 per cent of moisture is placed in a steeping tank followed by 90.7 cubic feet (6330 pounds) of 11 per cent sodium hydroxide solution containing 696 pounds of sodium hydroxide from a steeping liquor storage. The consistency of the batch is 9.6 per cent. The pulp is allowed to steep for 2 hours at a temperature of 20 C. The drain on the steeping tank is then opened and efliuent allowed to fiow continuously while the pulp is washed. The wash liquors are added at such a rate that the liquid level in the tank remains substantially constant. The first wash liquor is taken from a first wash liquor stor 10.6 per cent and containing 327.4 unds of sodium hydroxide. The first portion collected is dialyzed and there is obtained from the dialyzer 44.8 cubic feet (2947 pounds) of a purified 5 per cent solution of sodium hydroxide containing 147 pounds of sodium hydroxide. The purified solution from the dialyzer is returned to the first wash liquor storage and used in washing a subsequent batch of steeped pulp. The waste liquor from the dialyzer contains 180.4 pounds of sodium hydroxide.

The second portion of eiiluent collected from the steeping tank consists of 84.7 cubic feet (5788 pounds) of solution having an average sodiiim hydroxide content of 7.77 per cent and containing 449.6 pounds of sodium hydroxide. The second portion of eflluent is enriched with 480 pounds of 50 per cent sodium hydroxide solution to produce 6268 pounds of solution containing 11 per cent of sodium hydroxide. The enriched solution is returned to the steeping liquor storage for use in steeping a subsequent batch of pulp. The third portion of efiluent collected from the steeping tank consists of 97.0 cubic feet (6186 pounds) of a solution having an average sodium hydroxide content of 2.15 per cent and containing 133 pounds of sodium hydroxide. The third portion of efiluent is returned to the second wash liquor storage for use in washinga subsequent batch of steeped pulp. The washing of the pulp in the steeping tank is continued using water until it is substantially free from sodium hydroxide and the remaining efiiuent containing about 67.1 pounds of sodium hydroxide is run to waste. The washed pulp is then sheeted and dried and there is thus obtained 632 pounds of purified pulp (oven dry basis) containing 97.6 per cent of alpha-cellulose, 0.72 per cent of beta-cellulose, 2.0 per cent of gamma-cellulose, and 0.6 per cent of pentosans.

The sodium hydroxide in the three recycled portions of effluent amounts to 74.7 per cent of that contained in the steeping liquor, the first wash liquor and the second wash liquor. The sodium hydroxide run to waste consists of 180.4 pounds from the dialyzer and 67.1 pounds in the final washings of the pulp, or a total of 247.5 pounds which is 36 per cent of the weight (oven dry basis) of the unpurified pulp used. The yield of purified pulp is 92 per cent based on the weight of raw pulp treated.

We claim:

1. In a cold purification process for the production of at least 97 per cent alpha-cellulose from wood pulp, the steps which consist in: steeping chipped pulp at from 7 to per cent consistency with a unit volume of aqueous sodium hydroxide of from 8 to 15 per cent concentration; displacing a first efiiuent liquor consisting of about one-half unit volume of the steeping liquor with a first wash liquor of from 4 to 6 per cent sodium hydroxide; providing a second wash solution of sodium hydroxide of from 1.5 to 2.5 per cent concentration and displacing therewith about one unit volumeof a second eiliuent liquor from a steeped and once washed batch of pulp; displacing with about one unit volume of water the said second wash liquor as a third eiliuent liquor from a steeped and twice washed batch of pulp; and washing with water until substantially free of sodium hydroxide, and discarding further efiluent from the so-purified pulp, while: recovering from the said fi st efiluent by dialysis and dilution, about one-half unit volume of sodium hydroxide solution of from 4 to 6 per cent concentration and cycling this to serve as the first Wash liquor for another batch of steeped pulp; cycling the said second efliuent liquor for use together with added sodium hydroxide as the steeping liquor for another batch of unpurified pulp; cycling the said third efiluent containing from 1.5 to 2.5 per cent sodium hydroxide for use as a second wash liquor for a steeped and once washed batch of pulp; and repeating the cycle on successive batches while recovering purified pulp as the final step in the process.

2. In a cold purification process for the production of at least 97 per cent alpha-cellulose from wood pulp, the steps which consist in: steeping chipped pulp at from 10 to 12 per cent consistency with a unit volume of aqueous sodium hydroxide of from 10 to 13 per cent concentration at a steeping temperature of from 0 to 30 C.'for from 0.5 to 3 hours; displacing a first eiiluent liquor consisting of about one-half unit volume of the steeping liquor with a first wash liquor of from 4 to 6 per cent sodium hydroxide; providing a second wash solution of sodium hydroxide of from 1.5 to 2.5 per cent concentration and displacing therewith about one unit volume of a. second efliuent liquor from a steeped and once washed batch of pulp; displacing with about one unit volume of water the said second wash liquor as a third efiluent liquor from a steeped and twice washed batch of pulp; and washing with water until substantially free of sodium hydroxide, and discarding further eflluent from the so-purified pulp, while: recovering from the said first effluent by dialysis and dilution, about one-half unit volume of sodium hydroxide solution of from 4 to 6 per cent concentration and cycling this to serve as the first wash liquor for another batch of steeped pulp; cycling the said second effluent liquor for use together with added sodium hydroxide as the steeping liquor for another batch of unpurified pulp; cycling the said third eiliuent containing from 1.5 to 2.5 per cent sodium hydroxide for use as a second wash liquor for a steeped and once washed batch of pulp; and repeating the cycle on successive batches while recovering purified pulp as the final step in the process.

WILLIAM R. COLLINGS. RICHARD D. FREEMAN. MARTIN J. ROBERTS. WILLIS O. HISEY. 

